The present invention relates to a magnetic disk apparatus, and in particular, to a magnetoresistive head for use in reproducing magnetically recorded information.
With advancement of more compact and high-density magnetic disk apparatus, a magnetoresistive head (MR head) which is capable of producing a high reproducing output voltage irrespective of a relative speed between the disk and head has been put to actual use. An MR head presently in use mounted on magnetic disk apparatus utilizes anisotropic magnetoresistance effect in which its electric resistance changes depending on a relative angle formed between the direction of magnetization in a magnetic film and the direction of a signal detection current flow. Efforts to enhance its performance are being made through improvements of head structures and thin film materials. When a high surface recording density as high as several Gb/in.sup.2 is required, it is anticipated for any MR head which utilizes the anisotropic magnetoresistance effect that there will occur a decrease in sensitivity, therefore, R&Ds of a new head utilizing a macromagnetoresistance effect are under way in which its electrical resistance is caused to change responsive to a relative angle to be formed between respective directions of magnetization in two magnetic thin films which are laminated with a non-magnetic conductive thin film interposed therebetween. In any types of MR heads above, changes of electrical resistance take place due to rotation of magnetization in the magnetoresistance film, therefore, in order to obtain a noise-free reproducing waveform, movement of domain walls must be suppressed as much as possible.
As means for suppressing Barkhausen noise due to movement of the domain walls, a laminated structure of prior art is disclosed in U.S. Pat. No. 5,018,037 in which a hard magnetic thin film is laminated on the magnetoresistance film via a non-magnetic thin film, and other structures are disclosed in U.S. Pat. Nos. 5,018,037 and 5,079,035 in which hard magnetic thin films are disposed abutting the magnetoresistance film on both sides thereof.
For any hard magnetic thin film to be used in the magnetoresistance head, there are required two fundamental magnetic properties in order to prevent Barkhausen noise. One is that it must have a large coercive force. Namely, since the MR head is applied a signal as a magnetic field from a recorded medium and also subjected to a recording magnetic field from the recording head, thus, in order to ensure a stable reproducing characteristic to be maintained even when such external magnetic fields are applied, a coercive force of a sufficient magnitude is required so that a longitudinal bias magnetic field impressed from the hard magnetic thin film to the magnetoresistance film will not change easily. The other requirement is that an in-plane component of magnetization should be large enough, that is, squareness of a magnetization loop measured by applying a magnetic field along an intraplane direction should be large. Since it is the in-plane component among magnetization components by the hard magnetic thin film that plays a major role to act effectively as the longitudinal bias magnetic field, it is necessary for this in-plane component to be substantially large as well as for the squareness of its magnetization loop to be substantially large so that the longitudinal bias magnetic field will remain invariant even if external magnetic fields are applied.
FIG. 15 is a schematic diagram indicative of the structure of a prior art MR head disclosed in U.S. Pat. No. 5,005,096. This prior art MR head is directed to suppressing Barkhausen noise in magnetoresistive effect layer 15 by impressing a magnetic field produced by hard magnetic thin film 26 which is formed on non-magnetic underlayer 251 made of Cr or the like. Although it is possible to obtain hard magnetic thin film 26 which has a large coercive force and a large squareness through the provision of non-magnetic underlayer 251, since a portion of the magnetic field derived from hard magnetic thin film 26 is caused to recirculate through an MR element including soft magnetic thin film 13, non-magnetic conductive thin film 14 and magnetoresistive effect film 15, directions of magnetization in magnetoresistive effect film 15 become opposite between the sense area and both sides thereof as indicated in FIG. 15. Therefore, the state of magnetization in magnetoresistive effect film 15 becomes very unstable, thereby it becomes difficult to suppress Barkhausen noise.
FIG. 16 depicts the structure of an MR element disclosed in U.S. Pat. Nos. 5,018,037 and 5,079,035, in which on the both sides of the MR element a hard magnetic thin film is formed in order to eliminate a region having a magnetization component the direction of which is opposite within the magnetoresistive effect film such that the magnetization due to the hard magnetic thin film is effected to act only on a unilateral direction. This structure of laminated films including soft magnetic thin film 13, non-magnetic conductive thin film 14 and magnetoresistive effect film 15 (hereinafter referred to as soft magnetic thin film/non-magnetic conductive thin film/magnetoresistive effect film) is formed by the steps of etching other regions except for the sense area, forming hard magnetic thin films 26 on the both sides of the sense area, and forming electrodes on hard magnetic thin films 26.
Then, a further etching becomes necessary to etch the sides of the lamination of the soft magnetic thin film/non-magnetic conductive thin film/magnetoresistive effect film to form a gradual inclination so as to maintain a predetermined magnetic coupling and electrical contact between the soft magnetic thin film/non-magnetic conductive thin film/magnetoresistive effect film and the hard magnetic thin film 26 and also the electrode, thereby, a portion of hard magnetic thin film 26 is formed along the gradual inclination of the soft magnetic thin film/non-magnetic conductive thin film/magnetoresistive effect film. However, there is a problem associated with this prior art that since soft magnetic thin film 13 or magnetoresistive effect film 15 normally has a crystal structure of a face-centered cubic lattice, a portion of the hard magnetic thin film which is formed on such crystal structure tends to deteriorate its property greatly, in particular, its coercive force compared to that in other portion thereof.
Further, there is another problem associated with the prior art which generally uses Co--Cr--Pt hard magnetic thin film or Co--Cr hard magnetic thin film as hard magnetic thin film 26 that it is difficult to obtain an in-plane component of magnetization or a squareness which is sufficiently large on other areas excepting areas of soft magnetic thin film 13 or magnetoresistive effect film 15. Thin films generally have a tendency that the most dense crystal plane tends to grow parallel to the film surface thereof, thereby, in the case of the hard magnetic thin film of the prior art, plane (001) is likely to be oriented parallel to the film surface. On the other hand, since an easier magnetization direction is in the direction of &lt;001&gt;, magnetization tends to be directed perpendicular to the film plane, which causes the in-plane component which responds most effectively to the longitudinal bias magnetic field to decrease.
These problems associated with the prior art can be solved by providing an appropriate underlayer and forming a hard magnetic thin film thereon. According to current studies on magnetic recording medium, the provision of non-magnetic backing layers made of Cr or the like is known to be effective. However, the non-magnetic underlayer provided under the hard magnetic thin film for use in the MR head will interrupt a magnetically exchange coupling between hard magnetic thin film 26 and soft magnetic thin film 13 as well as magnetoresistive effect film 15, thereby, a desired effect to stabilize magnetization in the side regions of soft magnetic thin film 13 and magnetoresistive effect film 15 cannot be attained. Thereby, magnetization in these ferromagnetic thin films become unstable, thereby causing a Barkhausen noise and a variation in reproducing characteristics to occur readily.
Now, the problems associated with the prior art MR head using anisotropic magnetoresistive effect have been described hereinabove. However, the same problems will take place with an MR head which uses a macro magnetoresistive effect since its MR element is composed of a ferromagnetic thin film having a crystal structure of a face-centered cubic lattice.